1. Field of the Invention
The present invention relates to a ball valve of a trunnion mounting type etc. having retainers for ball seats and, more particularly, to a ball valve with an emergency sealing device which functions, in case of emergency such as when subjected to high temperatures due to a fire etc., to urgently seal off the portions of contact between the inner circumference of a valve body and the outer circumferences of retainers for ball seats with accuracy and also to accurately effect metal-to-metal sealing between the ball seat retainers and a ball.
2. Description of the Prior Art
A ball valve having ball seat retainers is generally provided with heat destructible, elastic seal members which are disposed one each between a ball and ball seat retainers and between the ball seat retainers and a valve body, thereby preventing a fluid from leaking out. However, when the ball valve is heated to high temperatures due to a possible accident such as a fire etc., the heat destructible seal members are burnt down or fused and lose their function. This brings about a possibility of a fluid leaking out of the valved pipes to thereby cause the fire to spread rapidly or cause the ball valve to fail to fulfill its operational function.
In order to avoid the aforementioned adverse phenomena, a conventional ball valve with an emergency sealing device comprises, as illustrated in FIG. 1, a ball 1, a ball seat 9, a ball seat retainer 2 having a metal-contact portion 3 formed on the leading end thereof on the ball 1 side, a retainer gland 8 disposed behind the ball seat retainer 2, a heat destructible spacer 6 interposed between the ball seat retainer 2 and the retainer gland 8, an O ring 10 for sealing off the space between the ball seat retainer 2 and a valve body 4, a coiled spring 7 accommodated in a hole 4' formed in the valve body 4 (or a body cap) and adapted to bias the retainer gland 8 toward the ball 1 side, and heat resistant flexible sealing ring 5 interposed between the outer circumference of the ball seat retainer 2 and the inner circumference of the valve body 4. In another prior art ball valve with an emergency sealing device as shown in FIGS. 2(A) and 2(B), a coiled spring 7 is retained by a spring holder 8' instead of providing a hole for the accommodation of the coiled spring 7.
When seal members, i.e. the ball seat 9 and O ring to 10, and the spacer 6 in the state shown in FIG. 1 or FIG. 2(A) are burnt down or fused, in case of emergency, either a piston action of the ball seat retainer 2 resulting from the difference between fluid pressures exerted on the front and rear annular end surface of the ball seat retainer 2 which have a thickness A corresponding to the difference in radius between a circle defined by a line of contact between the ball seat retainer 2 and the valve body 4 and a circle defined by a line of contact between the ball seat retainer 2 and the ball 1 as illustrated in FIG. 2(B) or the force of the coiled spring 7, or both, brings the metal-contact portion 3 of the ball seat retainer 2 into contact with the surface of the ball 1 and, at the same time, causes the sealing ring 5 to be pushed toward the ball 1 side through the retainer gland 8 to fulfill its sealing function in cooperation with the force of the coiled spring 7, as shown in FIG. 2(B).
In the prior art ball valves described above, the piston action resulting from the aforementioned difference in fluid pressure exerted on the ball seat retainer 2 is larger than the piston action before the ball seat 9 is burnt down or fused as illustrated in FIG. 1 or FIG. 2(A), i.e. the piston action of the ball seat retainer 2 resulting from the difference between the fluid pressures exerted on the front and rear annular end surfaces of the ball seat retainer 2 which have a thickness A' corresponding to the difference in radius between a circle defined by a line of contact between the ball seat retainer 2 and the valve body 4 and a circle defined by a line of contact between the ball seat 9 and the ball 1. However, since the frictional resistance of the sealing ring 5 relative to the valve body 4 in the state assumed after the spacer 6 has been burnt down or fused is larger than that of the O ring 10 before the spacer 6 is burnt down or fused, the force for urging the ball seat retainer 2 toward the ball 1 becomes insufficient and often causes incomplete contact between the metal-contact portion 3 of the ball seat retainer 2 and the ball 1. For this reason, a ball valve with an emergency sealing device is required to sufficiently increase the piston action of the ball seat retainer 2 resulting from the difference between the fluid pressures exerted on the front and rear annular end surfaces of the ball seat retainer 2 which have the aforementioned thickness A.
The conventional ball valve shown in FIG. 1 requires a boring process for forming a number of holes in the valve body 4 for accommodating therein coiled springs 7, which is troublesome work necessitating much time and labor. Although the conventional ball valve shown in FIG. 2(A) does not require such a boring process relative to the valve body 4, it requires preparatory formation of the spring holders 8' as separate component parts which increases the number of the component parts required by the number of the spring holders 8' to be prepared and also necessitates attachment of the spring holders 8' to the inside of the valve body 4 for accommodating therein the springs 7 which makes the work of assemblage complicated.